Who will benefit from reading this article?Psychiatrists, psychologists, primary care physicians, nurse practitioners, and other health care professionals. To determine whether this article meets the continuing education requirements of your specialty, please contact your state licensing and certification boards.

Since the inclusion of the borderline personality disorder (BPD) diagnosis in DSM, there have been multiple efforts to recast the disorder as part of an Axis I illness category. While the initial focus was on the schizophrenia spectrum,1 more recent authors have attempted to link BPD to mood disorders. There is considerable literature on the relationship between major depressive disorder (MDD) and BPD, and although the current understanding posits distinct disorders, overlapping biological underpinnings do exist.2 Attention has now turned to bipolar disorder, with several vocal advocates who propose reclassifying BPD as bipolar spectrum disorder.3,4 This article discusses the overlapping phenomenology of bipolar disorder and BPD and highlights distinguishing features of clinical diagnosis and treatment.

PrevalenceAccording to DSM-IV-TR, the prevalence of BPD is estimated at 2% of the general population, compared with 1% to 2% for bipolar disorder. Other estimates are closer to 5% for bipolar spectrum disorder.5 Depending on the population studied, there are varying estimates of the co-occurrence of BPD and bipolar disorder. In a recent comprehensive review by Paris and colleagues,6 the rate of bipolar I disorder in BPD patients ranged from 5.6% to 16.1%, with a median of 9.2%. The rate of bipolar II disorder was only slightly higher, 8% to 19%, with a median of 10.7%. The 2 studies with the strongest methodologies that used structured diagnostic interviews with adequate sample sizes and a 6- to 7-year follow-up showed a low rate of new onset of bipolar disorder in patients with BPD, with no difference from the comparison groups.7,8 A recent study that used the large Collaborative Longitudinal Study of Personality Disorders (CLPS) database, however, showed an increased rate of bipolar I and II disorders in patients with BPD compared with patients who had personality disorders other than BPD, including schizotypal, avoidant, and obsessive-compulsive personality disorders (19.4% and 7.9%, respectively). In addition, BPD patients had a higher rate of bipolar I and II disorder onset (8.2% for BPD vs 3.1% for the other personality disorders) over 4 years.9 While these studies suggest a moderately increased risk for bipolar disorder in patients with BPD, it was not nearly as high as the risk for MDD or substance abuse.

The rate of BPD in patients with bipolar I disorder varies from 0.5% to 30%, with a median of 10.7%, while in patients with bipolar II disorder, the rates are 12% to 23%, with a median of 16%.6 The relationship of BPD and cyclothymia has been examined in 1 study, and the results revealed exceptionally high comorbidity rates with BPD of 62%.10 However, while elevated rates of comorbid personality disorders have been found in patients with bipolar disorder, no differences between rates of BPD and the other personality disorders studied have emerged.6 These findings suggest that while BPD and bipolar disorder can co-occur, in general, comorbidity is not common.

DiagnosisDiagnosis of bipolar disorder or BPD can be difficult, because both can present with affective instability, irritability, and impulsivity. A comparison of DSM-IV-TR criteria is displayed in Table 1 and demonstrates considerable overlap.

The phenomenology of mania differs significantly from that of BPD. Factor analyses of manic symptoms have identified psychic and motor acceleration, psychosis, and irritability.11,12 A factor analysis and subsequent replication study revealed 3 factors for BPD: disturbed relatedness, behavioral dysregulation, and affective dysregulation.13,14 However, a number of recent studies have shown that the BPD factors correlate so highly with one another (with correlation coefficients of 0.92 to 0.98) that the factor analyses actually support a single overarching BPD construct.15-17

Recent studies that explored the overlap of BPD and bipolar disorder have outlined sever­al parameters to distinguish the 2 diagnoses9,18,19:

• Quality of mood episodes• Types of impulsivity• Longitudinal course

Symptoms such as irritability and quality of de­pres­sion have not proved helpful.